Frequency stabilizing device



April 19, 1938. UTTLE 2,114,846

FREQUENCY STABILIZ ING DEVICE Filed Nov. 29, 1935 4a 47 lnsu/af/onWITNESSES: INVENTOR Dona/d 'E'TORINEY BYW Patented Apr. 19, 1938 UNITEDSTATES PATENT OFFICE FREQUENCY STABILIZING DEVICE PennsylvaniaApplication November 29, 1935, Serial No. 52,113

Claims.

My invention relates to an oscillation generator, more particularly tomeans for stabilizing the frequency thereof.

It is an object of my invention to provide an oscillation generator inwhich frequency variation due to temperature changes shall be materiallyreduced.

Another object of my invention is to improve the frequency stability ofoscillation generators by the addition of a compensating element whichshall respond immediately to any tendency toward a frequency change ofthe oscillator.

A further object of my invention is to provide a frequency stabilizingdevice for oscillators which may be operated in any oscillator embodyingelectron discharge devices.

An additional object of my invention resides in the method forcompensating for undesired frequency changes occurring in oscillationgenerators.

Further objects of my invention will be disclosed in the followingdescription of the same, taken in connection with the accompanyingdrawing, wherein Figs. 1, 2 and 3 represent an oscillation generatorcircuit of the well known Hartley type, in each of which circuits adifferent species of my invention is embodied; and,

Fig. 4 is a view in perspective of one form which my compensating devicemay take.

In general, my invention comprises the idea of utilizing an elementhaving high temperature coefficient response to vary a compensatingcapacitor in an oscillation generator, and exposing said element to thatportion of the oscillator wherein the high frequency currents flow, insuch a manner that the heat produced in the temperature responsiveelement shall, for all practical purposes, be strictly in accordancewith the current flowing in these circuits.

My invention further embodies the design of the high temperaturecoeflicient element so that it will respond in a manner proportional tothe changes taking place in the frequency determining elements of theoscillators or more practically speaking, in accordance with changesoccurring in the tank coil, since the coil is mainly responsible for thefrequency shift of an oscillator due to temperature variations.

It has been known in the prior art to broadly utilize temperatureresponsive elements to compensate for frequency changes in anoscillation generator due to change of temperatures in and around thefrequency determining elements. In

the known circuits, however, the frequency responsive element wasdisposed in such a position that it received its heat by radiation, orconvection of the heat generated in the elements of the circuit as wellas from the heat conducted into the vicinity from the outside.

The frequency of a resonant circuit is affected by changes in physicaldimensions brought about by changes in temperature of the frequencydetermining elements of the circuit. Such changes in temperature arepractically due entirely to (first) power losses in the coil, whichlosses are a function of the current, and (secondly) variations inambient temperature.

Bimetallic compensating devices utilized in the manner stated cannotoperate efficiently nor provide sensitive control of the frequency of anoscillator, and an analysis of its operation discloses as the reasonthereof, that the compensating devices do not respond equally well fortemperature changes of the circuit elements caused by power losses inthe circuit as for temperature changes caused by variations in ambienttemperature. A fifty degree rise in the temperature of the frequencydetermining elements might for instance produce a temperature effect onthe compensator equivalent, say, to only a five or ten degree rise inambient temperature, and obviously under such conditions, thecompensator cannot exert a compensating effect which will be an accuratemeasure of the temperature of such elements.

It is further apparent that where'the heat, originating by power lossesin the circuit, must arrive at the compensating element by way ofconvection or radiation, an appreciable time lag must ensue between thetime that the heat is generated at its source, and the time at which itproduces a response in the temperature responsive element. It will befurther apparent that should the heat thus developed, fluctuate, asmight happen when the operation of the oscillator is a periodic one, thefrequency of the oscillator will never attain a condition of stability;in that, due to the time lag referred to, the frequency of theoscillator will fluctuate in accordance with the operation fluctuations.The cause and effect do not occur close enough in time to approach theideal conditions for compensation.

Since the power losses in the resonant circuit are a function of thecurrent flowing therethrough, changes in heat generated in any elementcarrying this current will be a measure of a change in temperature ofthe circuit caused by a change in the current flowing therethrough. I

make use of this fact in obtaining compensation in an oscillator whichwill be efficient, sensitive and accurate.

In accordance with one embodiment of my invention, I provide acompensating condenser at least one element of which is of a temperatureresponsive material, preferably of the bimetallic type, this elementbeing provided with terminals at distributed locations and connected inas a part of the oscillatory circuit which carries the high frequencycurrent. The heat produced in the temperature compensating element willbe the direct result of the oscillatory current flowing through thiselement, thus providing assurance that the heat produced in thecompensating device will occur simultaneously with the heat developed inthe oscillator circuit itself, by reason of the flow of the oscillatorycurrent therethrough. The question of a time lag therefore ispractically eliminated and the compensating effect of the compensatingcondenser will occur practically instantaneously and, therefore, willcompensate at the instant at which the compensation is necessary, toobtain a resulting stabilized frequency of oscillations in theoscillator, and not after an elapsed interval of time.

The bimetallic element must furthermore be so designed that for eachdegree change in the temperature of the frequency determining elementscaused by power losses, its temperature must change an amount equal tothat which would occur, had the degree change in temperature of thefrequency determining elements been brought about by a variation inambient temperature. The resistance of the bimetallic element will notappreciably govern its characteristics in response to ambienttemperatures, but will materially alter its response characteristicswhen responding to heat developed from electric current flowingtherethrough, and consequently its size and shape must be such as tooffer the proper resistance to current flow, which is necessary tocorrelate its response to current changes with changes in ambienttemperature This condition will be satisfied to a high degree if thebimetallic element be so designed that the ratio of the 1 R lossesdeveloped therein to the mass of the element times its specific heat, ismade equal to the ratio of the PR losses of the coil to the coil mass,times its specific heat. Expressed as a formula;

1 R Mass Specific heat Mass X Specific heat of bimetallic element. Whenso designed, the bimetallic element will respond in proportion to thechanges simultaneously occurring in the coil, regardless of whether suchchanges in the coil are brought about by changes in the oscillatingcurrent or load, or changes in ambient temperature.

To maintain a compensated condition, it is essential that for the steadycondition of operation of the oscillator, the design of the bimetallicelernent must also be such that the ratio of the PR losses developedtherein to the heat lost by radiation and convection must equal theratio of the heat developed in the coil during a steady condition, tothe heat lost from the coil by radiation and .convection.

With both of the above conditions satisfied, the compensating condenser,of which the bi metallic element comprises a controlling element indetermining its instantaneous capacity, will vary capacity in proportionto the changes of coil= PR in inductance occurring in the coil, whateverthe causes of such inductance changes may be.

In general the spacing of the compensating condenser should be largecompared to its movement so that the capacity change per unit change oftemperature will approximate a straight line. If then the change of coilinductance per unit change of temperature is a straight line thesequantities may be made equal and opposite to effect exact compensation.Obviously, other than straight line relations may be utilized providedthat th coil and compensator effects are equal and opposite at anytemperature within the limits designed for.

The changes in the compensating condenser will then be pro sortional tothe changes in the inductance of the coil, the constant ofproportionality being equal to the ratio of the total original capacityof the tank circuit divided by the original inductance of the coil.

In Fig. 1, I have disclosed a conventional oscillation generator circuitof the Hartley type embodying an electron discharge device I having grid3, cathode 5 and anode l electrodes, the grid and anode electrodes beingconnected to the extremities of a resonant circuit 9 comprising aninductor ii shunted by a condenser l3. A grid leak l5 and condenser llare connected in the lead to the grid, and a blocking condenser 29 inthe lead to the anode, anode potential being supplied to the plateelectrode through a choke coil ill, the blocking condenser referred to,preventing direct-current potential from be-- ing applied through thetank coil to the grid.

In accordance with the embodiment of my invention as described above,the bimetallic plate 23 of a compensating condenser 25 is connected inthe oscillatory circuit, in series with the tank condense l3, forexample. The oscillating current fiowing in the tank circuit will,therefore, have to flow through the bimetallic plate. The other plate llof the compensating condenser may be connected to the other side of thetank condenser, thus placing the compensating condenser effectively inshunt to the main tank condenser. Thus, should the load on theoscillator change to such an extent as to increase or decrease theoscillatory current fiowing in the tank circuit which in the normalcourse of events, would alter its frequency, the bimetallic condenserplate 23 will immediately respond one way or the other to the change inthe current flowing therethrough, and immediately change thecompensating capacity in shunt t the main condenser. The resonantfrequency of the tank circuit will thereby be effectively maintained atits desired value and the frequency of the generator will remainpractically constant.

In the circuit of Fig. 2, the compensating means also comprises acondenser 25 of the variable type, the adjustment of which ismechanically con trolled by the movement of a bimetallic element 29. Thebimetallic element is connected directly in the current path of theoscillator current in the tank circuit of the oscillation generator, andits movement in response to heat changes brought about by changes in theoscillatory current flowing in the tank circuit will control the valueof the compensating capacitor, which may be connected in shunt to themain tank-condenser in a manner somewhat similar to that of Fig. 1.

In the circuit of Fig. 3, we also have a compensating condenser 25 inwhich the adjustment is controlled mechanically by the movement of abimetallic element 3|. This modification, however, differs from that ofFig. 2 in that the bimetallic element is not connected directly in themain portion of the tank circuit, but practically the same efiect isobtained by means of a resistor 33 connected in the circuit and adjacentto or wrapped around the bimetallic element 3|. The resistor nowconstitutes a portion of the tank circuit where heat in concentratedform will be developed by the high frequency currents flowingtherethrough. By placing the bimetallic element adjacent to the resistorwhere the heat is concentrated, it will become very sensitive to changesin high frequency current values and practically the same effect andspeed of compensation can be obtained, as if the bimetallic elementitself were connected in the main tank circuit and conducted theoscillatory current therethrough.

In designing the bimetallic or heat responsive element of thecompensating condenser, if we provide per unit length of both bimetallicmaterial and tank circuit coil conductor, surfaces equal in area and oflike radiation characteristics, make the mass times specific heat thesame, and make the respective radio frequency resistance the same,conditions will be ideal for exact compensation for both transient andsteady state conditions; that is if currents of similar value are madeto flow through both coil and compensator. Where a different value ofcurrent or equivalent heat loss is developed in the compensator, thevalues of the physical and electrical characteristics of the bimetallicelement may be changed. It is only necessary that the various ratiosmentioned above be maintained in order to obtain idea compensatingresults.

An approach to the ideal condition may be realized by employing abimetallic or heat responsive element of convenient dimensions andloading the same by adding thermal mass to the element. This may be doneby soldering blocks of copper or other suitable metallic material to thebimetal element, the size and shape of the blocks being such as willapproximate the ratio of PR losses in the element to its mass timesspecific heat, and that ratio of the PR. losses in the element to itslosses by radiation and convection, which are necessary for matching theoperating characteristics with the changes occurring in the coil.

One practical form of a compensating condenser which my invention mayassume, is illustrated in the device of Fig. 4 and it comprises twoplates 35 and 31, one circular in shape and adjustably supported bymeans of a threaded bolt 39 through a supporting wall 4| of conductivematerial mounted on a base member 43 of insulating material. The otherelement or plate 31 of the condenser comprises the temperatureresponsive element which, for efiicient operation, should consist of abimetallic plate. In the form shown by me, it comprises a platerectangular in shape and provided with a slot 45 through the centralportion extending from one edge to a point near the opposite edge,leaving two large sections 41' and 48 joined by a strip 5| at the top.The plate can be mounted in upright position on the base 43 by attachingit to terminal lugs 53 and 55 of conductive material also mounted on thebase. The capacity of the condenser may be adjusted manually by varyingthe spacing between the circular plate 35 and the bimetallic element byreason of the threaded engagement of the circular plate in itssupporting wall.

The sensitivity of the device, on the other hand, can be varied bychanging the size of the slot 45 in the bimetallic element. While onlyone slot has been shown in the plate 31, the resistance of the plate maybe increased by providing additional slots extending alternately fromboth the upper and lower edges thereof.

The direction of compensation can be controlled by mounting thebimetallic element with either one face of the other, facing thecircular plate, whereupon the capacity of the compensating condenser maybe made to increase or decrease upon heating of the bimetallic plate.

The size of the terminal blocks 53 and 55 will depend upon the amount ofthermal mass and radiation surface found necessary to be added to thebimetallic element to match it with the coil in the tank circuit.

The above described compensating condenser is applicable in the circuitarrangement of Fig. 1. Where, however, it is desired to utilize theembodiments of Figs. 2 and 3, an additional element will be added to theconstruction illustrated in Fig. 4. This additional element willcomprise a plate in capacitive relationship to the circular plate shownand it will be mechanically coupled to the bimetallic element which willnow control the movement of the added plate, and not constitute aportion of the compensating capacity.

Once the bimetallic element has been shaped and slotted, it will not bedesirable to alter its size of shape to change its sensitivity. In fact,the sensitivity could be changed in one direction only and that would beby enlarging the slot or providing additional slots as pointed outabove. Should the design of the circuit or other factors necessitate adecrease in the sensitivity of the compensating condenser, the resultcan very easily be accomplished by connecting a shunt 51 across theterminal blocks, whereby a portion of the oscillator current, whichwould normally go into heating up the bimetallic plate will be shuntedaround it and the eflect on the bimetallic element will be reduced asdesired.

The compensating means of my invention responds effectively to bothchanges in ambient temperature and changes in load, and when onceadjusted to compensate for changes in circuit constants due to current,it will at the same time compensate for such changes when due tovariations in ambient temperature, to maintain a stabilized frequencycondition of the oscillator. Should either the load condition or theambient temperature remain constant, it is apparent that compensationwill still occur due to changes in the other, which might tend todisturb the fre quency equilibrium of the circuit.

Should the frequency stability requirement for any particular circuit beso stringent as to require taking into consideration such comparativelyminor efiects on the frequency as .might be attributed to temperatureefiects on the tank condenser and leads, etc., the compensatingcondenser may well be adjusted to take care of such changes, the mannerof adjusting said condenser having been pointed out above in thedescription of the device of Fig. 4.

While I have disclosed my invention in detail, it is apparent thatvarious modifications of the same would be apparent to one skilled inthe art and I, therefore, do not desire to be limited in my protectionto the details disclosed by me ex cept as may be necessitated by theprior art and the appended claims.

I claim as my invention:

1. In combination, an oscillation generator comprising inductance andcapacity for determining the frequency of oscillation thereof, saidinductor being normally heated due to oscillatory current in saidgenerator, means for compensating for frequency changes normally arisingout of changes in said current, said means comprising a compensatingcondenser including a bimetallic element, said bimetallic element beingcoupled into said oscillation generator to pass current in proportion tothe current flowing in said oscillation generator, and having a ratio ofheat developed therein by said current to the product of its mass timesits specific heat substantially equal to corresponding ratio withrespect to said inductance.

2. For use in combination with an oscillation generator to compensatefor frequency changes therein, a condenser having one plate ofbimetallic material and terminal connections at two points on saidwhereby current may be con-ducted along this plate to the exclusion ofthe other plate.

3. For use in combination with an oscillation generator to compensatefor frequency changes therein, a condenser comprising a bimetallicelement and terminal connections at two points on said bimetallicelement whereby current may be conducted therealong, said bimetallicelement constituting a plate of said condenser.

In combination, an oscillation generator comprising inductance andcapacity for determining the frequency of oscillation thereof, means forcompensating for frequency changes normally arising out of changes inload current said means comprising a condenser including a bimetallicelement, an impedance connected in circuit to carry current proportionalto the oscillatory current in said generator, sai-d bimetallic elementbeing disposed adjacent to said impedance and adapted to be effectivelyheated thereby.

5. In combination, an oscillatory circuit comprising an inductor, saidinductor having heat developed therein in accordance with current insaid circuit, and a frequency stabilizing device connected in circuittherewith and having heat developed therein in accordance with currentin said circuit, said frequency stabilizing device having a ratio ofheat developed therein to the product of its mass times its specificheatsubstantially equal to the ratio of heat developed in said inductor tothe product of the mass of said inductor times its specific heat.

6. In combination, an oscillatory circuit comprising an inductor and acompensating condenser, said inductor having heat developed therein inaccordance with current in said circuit, said compensating condenserembodying a bimetallic control element in series with said inductor andadapted to carry the current flowing through said inductor to developheat in said bimetallic element, said bimetallic element having a ratioof heat developed therein to the product of its mass times its specificheat substantially equal to the ratio of heat developed in said inductorto the product of the mass of said inductor times its specific heat.

7. In combination, an oscillatory circuit comprising an inductor and afrequency stabilizing device comprising a compensating condenser havinga controlling element of bimetallic material, both said inductor andcontrolling element having heat developed therein due to oscillatorycurrent in said circuit and thermal mass loading means embodied in saidfrequency stabilizing device to make a ratio of heat generated thereinto the product of its mass times its specific heat substantially equalto the ratio of heat developed in said inductor to the product of themass of said inductor times its specific heat.

8. In combination, an inductive and capacitive reactance connected toconstitute a tuned circuit, said reactances being adapted to have heatdeveloped therein in accordance with oscillatory current in said tunedcircuit whereby the frequency of said tuned circuit is prone to shiftupon a change in said current, said reactances having a certain ratio ofheat developed therein to their combined masses times their specificheats, a compensating reactance connected in circuit with said tunedcircuit for compensating for such shifts in frequency, said compensatingreactance also being adapted to have heat developed therein inaccordance with oscillatory current in said tuned circuit, saidcompensating reactance having a ratio of heat developed therein to itsmass times its specific heat which ratio is substantially equal to thatof said reactances combined.

9. In combination in a tuned circuit, an inductor adapted to have heatdeveloped therein due to oscillatory current in said tuned circuit, saidinductor having a certain ratio of heat developed therein to its masstimes specific heat, a condenser having a plate thereof temperatureresponsive and adapted to have heat developed therein in accordance withoscillatory current in said tuned circuit, said condenser having a ratioof heat developed therein to its mass times its specific heat whichratio is substantially equal to that of said inductor.

10. In combination with an impedance which changes in magnitude inresponse to temperature rise connected in an electric circuit, saidimpedance being adapted to have heat developed therein in accordancewith current flowing in said circuit, said impedance having a certainratio of heat developed therein to the product of its mass by itsspecific heat, a compensating means which responds to temperature riseconnected in said circuit and adapted to have heat developed in it inaccordance with current flow in said circuit and thereby to be alteredto compensate the change in said impedance due to said heat developedtherein, said compensating means having a ratio of heat developedtherein to the product of its mass by its specific heat which issubstantially equal to the ratio first mentioned.

11. In combination With an impedance which changes in magnitude inresponse to temperature rise connected in an electric circuit, saidimpedance being adapted to have heat developed therein in accordancewith current flow in said circuit, said impedance having a certain ratioof heat developed therein to the product of its mass by its specificheat, a compensating means which responds to temperature rise tocompensate the first-mentioned change connected in said circuit andadapted to have heat developed in it in accordance with current flow insaid circuit, said compensating means having a ratio of heat developedtherein to the product of its mass by its specific heat which issubstantially equal to the ratio first mentioned, the temperature changeproduced in said compensating means and in said impedance by a givencurrent flow in said circuit being substantially equal.

12. In combination with a first impedance and a second impedance ofopposite sign connected to constitute an electric circuit at least saidfirst impedance changing in magnitude in response to temperature rise,said impedances being adapted to have heat developed therein inaccordance with the current flowing in said circuit, said firstimpedance having a certain ratio of heat de veloped therein to theproduct of its mass by its specific heat, a temperature-responsivecompensating means connected in said circuit and adapted to have heatdeveloped therein in accordance with the current flow in said circuit,said compensating means being adapted to be so altered by a givenambient temperature rise as to change said second impedance in an equalpercentage but in opposite sense to the firstmentioned change, saidcompensating means hav ing a ratio of heat developed therein to theproduct of its mass by its specific heat which is substantially equal tothe first-mentioned ratio, and the temperature rises producedrespectively in said one impedance and in said compensating means by agiven current flow in said circuit being substantially equal.

13. In combination with a first impedance and a second impedance ofopposite sign connected to constitute an electric circuit at least saidfirst impedance changing in magnitude in response to temperature rise,said impedances being adapted to have heat developed therein inaccordance with the current flowing in said circuit, said firstimpedance having a certain ratio of heat developed therein to theproduct of its mass by its specific heat, a temperature-responsivecompensating means connected in said circuit and adapted to be heated bycurrent flow therein, said compensating means being adapted to be soaltered by temperature rise as to change said second impedance inopposite sense to the first-mentioned change, said compensating meanshaving a ratio of heat developed therein to the product of its mass byits specific heat which is substantially equal to the firstmentionedratio.

14. In combination in a tuned circuit, a vari able condenser, aheat-responsive element and means for causing it to vary said condenser,an inductor comprising a conductor, each unit length of said element andsaid inductor having equal areas and being of like heat-dissipationcharacteristics, the product of the mass by the specific heat of therespective said unit lengths being equal, and the radio-frequencyresistances of the respective said unit lengths also being equal.

15. In combination in a tuned circuit, a variable condenser, aheat-responsive element and means for causing it to vary said condenser,an inductor comprising a conductor, each unit length of said element andsaid inductor having equal areas and being of like heat-dissipationcharacteristics, and the radio-frequency resistances of the respectivesaid unit lengths also being equal.

DONALD G. LITTLE.

